Millimeter-wave and submillimeter-wave bands are important for radio astronomy and the measurement of molecules in the atmosphere. The improvement of a superconducting (SIS) mixer operating in these bands has been actively studied in astronomical observatories at many sites around the world, for example, in the National Astronomical Observatory of Japan, the National Astronomy Observatory of the U.S., and the German and French radio astronomy observatories.
Among signal detection technologies in the millimeter-wave and submillimeter-wave bands, the heterodyne method is the most popular method at present. In such a heterodyne system, the usage of efficiencies of upper and lower sidebands is closely related to the properties of the radio waves to be detected. In other words, the usage differs remarkably depending on whether the frequency characteristics of a radio wave source to be measured represent a continuous spectrum or a line spectrum. In the case of the continuous spectrum, a double-sideband receiver operates with higher efficiency. In the case of the line spectrum, a single sideband receiver is preferably used.
In ground measurement, a receiver receives radiated components released in the atmosphere as noise. Accordingly, extra caution is required in line spectrum observation using the double-sideband receiver. The reason is that spectra of the air are not always uniform and oxygen and water molecules give high noise temperatures to the spectra of the molecules to be determined.
In the millimeter-wave and submillimeter-wave bands where the spectra of many molecules are concentrated, it is worldwide common sense that a superconducting (SIS) receiver is generally used as the most sensitive receiver. SIS mixers are being developed in order to realize observation using the SIS mixers in a wider band (a 30% to 40% wider band: the band is determined by waveguides).